Such detectors are used in particular for detecting the deterioration of wear members, for example motors such as reactors, and/or instances of malfunction, producing the separation of metallic particles which are entrained in a flow of fluid such as a flow of oil or of refrigerant. These metallic particles are to be detected by the magnet of the detector, which can trigger an alarm for a predetermined quantity of particles. The detected particles are magnetizable particles, either because they derive from a metallic member, or because they originate from a non-magnetic material but which is secured to a magnetic material. The detection of the particles permits the preventive maintenance of the mechanical members by continuous control of their deterioration.
For high efficiency, such detectors must attract a very high proportion of the particles passing in their vicinity, even though the fluid flow is at high speed and when said fluid has a tendency to retain the particles, for example in the case of a viscous fluid such as an oil.
Moreover, the detected particles should be concentrated in the air gap to produce if desired an alarm, the filling up of the air gap closing an electrical alarm circuit.
Of course, these phenomena of detection differ according to the nature of the particles, their sizes, the nature of the carrier fluid and its speed of flow.
The known detectors of the type indicated in the introduction are constituted by a magnet surrounded at each of its two ends by a polar mass.
The magnet is a known Al-Ni-Co alloy.
The users of these known detectors have noted numerous drawbacks and deficiencies. In particular, the efficiency of detection of the particles, which is to say the ratio of the number of particles detected to the number of particles injected in the course of a trial (in %) is low, even for large particles (1000 .mu.m).
Moreover, the magnets used are subject to accelerated aging because of the high temperatures (of the order of 175.degree. C. for example for a motor oil), the aging being moreover permanent. Moreover, these magnets can be remotely demagnetized, for example to permit the use of a magnet to collect the detected particles.
Moreover, in the case of the use of the detector for triggering an alarm by closure of an electric circuit by the detected particles covering the air gap, it has been determined that the particles do not have the tendency to accumulate in the air gap, such that a very high total number of detected particles is necessary to trigger the alarm.